This invention relates to a method and apparatus for the generation of radar range gates. More specifically, the present invention relates to a range rate aiding device having the ability to autonomously control the range gate sampling of a non-stationary target over the course of multiple pulse intervals.
The function of most radars is to first determine the location of a target, often in spherical coordinate representations including its range (distance) and angles (direction). A target is defined broadly to include any object of interest that reflects energy back to the radar where it is capable of being detected. A pulsed radar system acquires information about objects of interest by transmitting bursts of electromagnetic energy and then processing the reflected signals. Successive pulses are transmitted at a pulse repetition frequency (PRF) giving rise to a train of pulses separated by a pulse repetition interval (PRI).
Once identified, the radar must keep the reflected object, or target, within a tracking range interval, or gate. The range of the target is determined by measuring the elapse time, or echo time, between the transmission of the radiation from the transmitter and its reception after being reflected from the target. The echo time is a function of the propagation distance from the transmitter to the target and back to the receiver. It is common, though not necessary, that a single antenna be used for both transmission and reception, in which case the echo time is given by 2R.sub.TR /c, where R.sub.TR is the distance between the target and the radar, and c is the speed of light. The echo time will vary with the relative location and motion of the radar and target.
In modern radar applications, the return signal is commonly processed by being amplified, filtered and demodulated to produce a video signal which refers to the echo signal after removal of the carrier frequency. In a digital system, the video signal is sampled repeatedly with an analog-to-digital (A/D) converter. A single sample of the digital video signal, acquired at a given delay interval after the transmitted pulse, is called a range gate. A searching radar may employ a series of range gates covering the entire pulse repetition interval in order to investigate the entire spatial region up to the maximal range, each range gate corresponds to a select point in space.
The signal data are conveyed to a processor for target identification and tracking purposes. The function of target identification is to distinguish between that portion of the transmitted pulse reflected back by the target from the extraneous portion of the echo; the extraneous portion being termed clutter. After identification and selection of a target, the relative distance of the target is tracked through successive pulse repetitions. Target tracking may involve updating the range gate delay interval as well as the number of range gates. It is common to allow a sampling window to cover one or multiple range gates. A tracker mechanism may either manually or automatically adjust the range gate delay to fix, or otherwise set firmly, the relative position of the echo with that of the digital sampling window. The position of the sampling window is continually updated to keep it centered on the target as the echo shifts temporally.
One circuit for range tracking is the split-gate range tracker which uses two gates straddled about the echo; an "early gate" to cover the initial portion and a "late gate" to cover the final portion. The range gate delay is properly chosen where the signal energy enclosed by each of the gates is the same. Any difference in energy provides a measure of the error, and can be used to modify the range gate delay.
Range rate aiding, also termed velocity aiding, refers to a technique used to adjust the range gate delay between pulse repetitions to compensate for the relative motion of the target. The objective is to minimize the variation between the echo time and range gate delay. The target tracking functions are frequently performed by electronic circuitry as opposed to software executed by a computer because the computer may not be capable of updating the range information at rates sufficiently high to maintain the sampling window centered on the moving target. Dedicated tracking circuitry has the advantage that it may independently track the target of interest after being provided with the range and velocity. With dedicated tracking circuitry, the computer need only periodically update the range information and account for acceleration.
Present methods of range rate aiding are excessively large or complex for missile applications and applications where the radar platform is small and rapidly mobile. One prior art implementation uses a tapped delay line wherein discrete delay intervals, each separated by a desired velocity aiding increment, are produced at the taps on such a line. Control of the velocity aiding is then determined by the particular tap selected.
A second velocity aiding technique is described in U.S. Pat. No. 4,156,875 to Keane in which the range gate delay is controlled by two clocks. The first clock provides coarse control and the second, running at a higher frequency, provides fine control. After receiving the initial range and velocity, the sampling electronics converts the estimated range to a number of cycles of the coarse clock. The delay represented by the coarse clock corresponds to a coarse delay, and is substantially equal to the estimated range. When a counter running off the coarse clock has finished counting off the coarse delay, a second counter running off the fine clock begins counting. The number of cycles of the fine clock is calculated such that the fine delay, when added to that of the coarse delay, causes the range gate to be positioned as close as possible to the estimated echo time. The coarse and fine delays are then modified at the beginning of each successive PRI.
The ability of the Keane apparatus to adjust finely the range gate is limited by the frequency of the fine clock. More specifically, the degree of control is proportional to the frequency of the clock. The attaining of the precision required by modern radars necessitates the use of clocks operating in the gigahertz range. Such clocks and the associated tracking circuitry pose formidable engineering challenges and may be prohibitively expensive for practical applications.